Who enforces booking class?
 

Just curious, but who enforces the booking class and how is it done?

DONEx books into D (of course) on segments with business class, and A on U.S. domestic two-class segments. I assume that U.S. domestic three-class segments are supposed to book into D, so if, hypothetically, some were somehow booked in A, would this be likely to cause a problem at check-in?

YMMV on this, but I would say is unlikely.

The main situation where I have wondered about this is if a US domestic sector on a 2 class aircraft (D booking into A), was changed so that it was now on a 3 class aircraft - would they realise that your "A" class seat come about because of the DONEx rule?
Probably not, if such flights existed, I would imaging...

During my LONE4 last year, I didn't have any cabin changes, but when I was shifting flights around (due to first round of BKK protests) and at check in, they changed my L booking class into a mix of S, O, N and Y (quite randomly as well).

From my experience, if the aircraft suddenly becomes 3 class from 2 class and are SOLD as 3 class, then you will definitely be downgraded back to J Class at checkin - the checkin agent will pick it up in the DCS.

As for Himeno, they actually didn't swap you class at all. what is displayed on the BP may not be the actual BP. for eg. on BA they use M Class for economy class for ALL ticket type on their BP.

They did. My TA at Qantas Travel confirmed that they (IB/BA) had. That, and I also got full Y points instead of the discounted points. I got points*1 instead of 0.25 like I would have under L class.

What's the DCS and how would it be used in this case? From the name, I can guess Departure Control System.

Rebooking for irrops normally is down in whatever booking class is required rather than necessarily keeping the original booking class. That is completely different situation to OP. I agree with others that you'd be unlucky to have a problem at check-in although of course you shouldn't bring attention to it yourself.

Very simple: if the reservation is in A and the underlying ticket matches, no one will even cast a glance in your direction. If there's a mismatch, it is still likely no one will check, but, if they do, you're screwed.